This invention relates to a thickness gauge for non-metallic materials in the form of foil, film, tape or the like.
The thickness of such materials can be measured indirectly by measuring the absorption of a radiation source, or directly by measuring and comparing the distance of the two faces or surfaces of the film. Gauges which directly measure the distance of the film surfaces can perform the measurement either with or without contact.
Document GB 2,217,835A describes a non-contact thickness gauge for measuring the thickness of foil, film and tape made of non-metallic materials.
In particular, it illustrates the combination of a first optical sensor which measures the distance to the upper surface of the film, and a second inductive sensor which measures the distance to a metal reference surface.
The inductive sensor is hollow to allow the housing of the optical sensor, and the two sensors are installed coaxially so that they perform their measurements along the same measuring axis.
While the measurements of the first sensor are performed directly on the measuring axis, those of the second sensor naturally take place on an area surrounding that measuring axis (typically an annulus), so that in practice the second sensor estimates the distance to the metal reference surface along that measuring axis.
In a first disclosed embodiment, the metal reference surface comes into contact with the lower surface of the film.
Since the film moves, the said reference surface is preferably a roller in order to minimise friction.
GB2217835A also describes a second construction in which the metal reference surface is positioned at a distance away from the lower surface of the film, such distance being known as a result of the use of a third, optical sensor.
The above-mentioned thickness gauge presents some drawbacks, however.
Depending on the composition of the material being examined and its surface characteristics, the ratio between the regular reflection factor and the diffuse reflection factor can vary considerably, as can the diffusion indicating curve.
The optical sensors used in thickness gauges designed in accordance with the known technique must therefore be specially constructed on the basis of the optical characteristics of the materials to be measured.
Equally, the optical characteristics of the materials to be measured often present intentional or unintentional dissimilarities which can prejudice the regulation system of the sensor.
In this respect, it should be noted that the speed of the material to be measured relative to the thickness gauge can reach values of around 10 m/sec. By contrast the time constant of the regulator of an optical sensor is measured in tenths of a second, with the result that sudden variations in the optical characteristics of the material overload the regulation circuit of the optical sensor. This makes the measurements performed unreliable because of the adjustment time required by the optical sensor regulator (typically a few tenths of a second). Moreover, the large size of currently known optical sensors means that inductive sensors with a very large diameter have to be used, thus reducing the reliability of the measurements taken.
In fact, the larger the size of the annulus on which the measurement of the inductive sensor is performed, the less precise is the estimated distance of the metal reference surface along the measuring axis.
It should be noted that optical distance sensors have a maximum accuracy of around .+-.10 .mu.m, which is not always satisfactory, especially when the average thickness of materials measured is under 1 mm.
In general the non contact thickness gauges currently known for non metallic materials present reliability problems.